1. Field of the Invention
The present invention relates to a high-frequency coil device and a method of manufacturing the same, and particularly to a high-frequency coil device of fine pitch for GHz and a method of manufacturing the same.
2. Description of the Related Art
A conventional high-frequency coil device will be described with reference to FIGS. 10A, 10B and 10C. Here, FIG. 10A is a cross-sectional view showing a conventional high-frequency coil device, FIG. 10B is a cross-sectional view taken along a line C—C of FIG. 10A, and FIG. 10C is a partially enlarged view of FIG. 10B.
As shown in FIGS. 10A, 10B and 10C, a spirally-shaped coil 32a formed of a convex-shaped Cu (copper) layer having a thickness of, for example, 15 to 25 μm is formed on a dielectric substrate 30 formed of polyimide resin having a thickness of, for example, 20 to 30μm. The surface of the spirally-shaped coil 32a is covered by an Au plating layer 36 having a thickness of 0.3 to 5 μm.
Here, the material of the dielectric substrate 30 is not limited to polyimide resin, and epoxy resin or phenol resin can be used.
A signal line 38 having the same structure as the spirally-shaped coil 32a is disposed so as to be adjacent to the spirally-shaped coil 32a. The surface of this signal line 38 is coated with the Au plating layer 36 as in the case of the spirally-shaped coil 32a. 
The center-portion of the spirally-shaped coil 32a (more accurately, the Au plating layer 36 coated on the surface of the coil 32a at the center portion) and the signal line 38 (more accurately, the Au plating layer 36 coated on the surface of the signal line 38) are connected to each other by an Au wire 40, thereby constructing a high-frequency coil device having such a structure that the spirally-shaped coil 32a formed of the convex-shaped Cu layer is formed on the dielectric substrate 30.
Next, a method of manufacturing the conventional high-frequency coil device will be described with reference to the cross-sectional views of FIGS. 11 to 15.
First, as shown in FIG. 11, the Cu layer 32 having a thickness of 15 to 25 μm is formed on the dielectric substrate 30 of polyimide resin having a thickness of 20 to 30 μm. Subsequently, as shown in FIG. 12, a resist film is coated on the Cu layer 32, and then the resist film is patterned in a spiral shape having a fine pitch by using the photolithography technique to form a resist pattern 34.
Subsequently, as shown in FIG. 13, the Cu layer 32 is selectively etched and removed by using the resist pattern 34 as a mask, and then the resist pattern 34 is peeled off as shown in FIG. 14. As described above, a coil 32a comprising the convex-shaped Cu layer 32 which is patterned in the spiral shape is formed on the dielectric substrate 30.
Subsequently, as shown in FIG. 15, Au (gold) plating treatment is carried out on the spirally-shaped coil 32a to coat the surface and side surface of the spirally-shaped coil 32a with the Au plating layer 36.
Finally, as shown in FIG. 10A, a wire bonding is carried out so that the central portion of the spirally-shaped coil 32a (more accurately, the Au plating layer 36 coated on the surface of the coil 32a at the center portion) and the signal line 38 formed simultaneously with the coil 18 in the same process (more accurately, the Au plating layer 36 coated on the surface of the signal line) are connected to each other by an Au wire 40.
As described above, a high-frequency coil device having a spirally-shaped coil 32a formed of a convex-shaped Cu layer 32 which is coated with the Au plating layer 36 is formed on the surface and side surface thereof.
In the above conventional high-frequency coil device, since the spirally-shaped coil 32a formed of the convex-shaped Cu layer 32 is formed by selectively etching the Cu layer 32 with the resist pattern 34 as a mask, the section of the coil 32a has a trapezoidal shape having inclined side surfaces as shown in FIG. 14 and FIG. 10C. Therefore, dispersion occurs in the sectional area, and thus the dispersion of the coil inductance is intensified.
That is, it has been difficult for the conventional high-frequency coil device to manufacture a high-frequency coil device for GHz which needs a fine-pitch coil having small dispersion in coil inductance.